1. Field of the Invention
The present invention relates to an apparatus which optically records and reproduces data.
2. Description of Related Art
In a conventional optical system for recording and reproducing data, divergent light emitted from a laser source is collimated by a collimating lens having a positive power, so that the collimated light is converged onto a data recording surface of a data recording medium by an objective lens. Light reflected by the data recording surface passes through the objective lens and is separated from the incident light path by a beam separator to converge onto a light receiving element. The light receiving element detects a focus error signal, a tracking error signal, and a data signal, etc., from the reflected light received by the light receiving element. In general, a relatively inexpensive semiconductor laser in which modulation can be easily carried out is used as the laser source. However, the wavelength of the laser beams emitted from the semiconductor laser varies due to a mode hopping phenomenon which occurs when the output power thereof varies. Consequently, if the power of the semiconductor laser changes between a recording operation and a reading operation, a change in the wavelength takes place.
Accordingly, it is necessary to correct chromatic aberrations while not causing a deviation of the focus on the data recording surface, even if the wavelength varies. To this end, in a conventional optical system, both the collimating lens and the objective lens are each subject to a correction of chromatic aberrations. However, the operation or treatment to correct the chromatic aberrations of both the collimating lens and the objective lens is rather expensive and increases the size of the entire lens system.
For example, in an arrangement proposed in Japanese Unexamined Patent Publication (Kokai) No. 62-269922, the objective lens is made of a single lens to reduce weight. Chromatic aberration caused in the objective lens is corrected by the collimating lens. However, in such an optical arrangement of an optical data recording and reproducing apparatus, the power of the objective lens is approximately three times the power of the collimating lens; a slight inclination or deviation of the optical axis of the collimating lens causes a considerable transverse chromatic aberration. Consequently, if there is a change in the wavelength, the deviation of a beam spot position on the data recording surface occurs. This makes it difficult to adjust the position of the collimating lens.
There is also known an optical system in which a beam separator is provided between the objective lens and the collimating lens to separate return light reflected from the data recording surface from light incident on the recording surface. The return light thus separated can be received by a light receiving element through a condenser lens. In this optical system, in the case where the condenser lens is made of a single lens, if the collimating lens has a large chromatic aberration, a change of a focus error signal occurs when the wavelength varies. Namely, resultant chromatic aberration of the collimating lens and the objective lens is corrected and a position of the objective lens is controlled in in-focus condition by a servo-controlled auto focus mechanism. When the wavelength varies, the control of the auto focus mechanism is effected such that the objective lens is out of focus. Because the return light reflected from the data recording surface is adversely affected by the chromatic aberrations of the objective lens, the focus error signal varies. To eliminate this drawback, the chromatic aberration of the condenser lens must be excessively corrected, similarly to the collimating lens, which requires an expensive operation.